(a) Field of the Invention:
This invention relates to improvements in a focusing device for microscopes.
(b) Description of the Prior Art:
In an objective lens of a microscope, in the case of a high magnification, the depth of focus will be so shallow that, if the objective lens is focused during the observation through an eyepiece, the focusing will be comparatively accurate. On the other hand, in the case of such low magnification as about 4 times as large, the depth of focus will be deeper than in the objective lens of a high magnification and therefore, even if the focusing is not accurate, an object image observed through the eyepiece will be able to be favorably observed by the adjusting action of the naked eye. However, in case it is photographed with a photographing device, the obtained photograph will not be favorable and will be often out of focus. In order to eliminate such defects, there is a method wherein an auxiliary telescope is added to magnify and focus the image. However, in this method, the operation is difficult and the precision is little improved. There is another method wherein the image is focused by locating a position in which no parallax with the focusing glass is produced by vibrating the eye of the observer in the plane vertical to the optical axis. This method is also hard to generally use and is not so recommendable.
FIG. 1 is a principle view for explaining the above mentioned latter method. In the drawing, the reference numeral 1 denotes an objective lens, 2 denotes an object to be observed, 3 denotes an object image and 4 denotes a light shielding plate having a pinhole 4a and arranged in front of the objective lens 1 so as to be rotatable around an axis 4b coinciding with the optical axis. The image 3 of the object 2 is formed through the pinhole 4a. In this case, in a focused plane Q.sub.1, even when the pinhole 4a is above the optical axis as shown in (A) and is below it as shown in (B) in FIG. 1, the image 3 will be formed in exactly the same position but, in a plane Q.sub.2 out of focus, an image 3' in the case shown in (A) and an image 3" in the case shown in (B) will be formed in different positions. Therefore, if the image position is adjusted so as not to shift even if the light shielding plate 4 is rotated, the focusing of the lens 1 will be adjusted to be correct.
A microscopic photographing device using such focus detecting device shall be explained in the following. FIG. 2 shows an example of an optical system of such microscopic photographing device. The reference numeral 11 denotes a light source, 12 denotes a collector lens, 13 denotes an aperture diaphragm of an illuminating system located in a pupil position P.sub.0 of the optical system, 14 denotes a condenser lens, 15 denotes a sample, 16 denotes an objective lens, 17 denotes an eyepiece, 18 denotes a beam splitter, 19 denotes a film surface, 20 denotes a relay lens, 21 denotes a reticle and 22 denotes a viewer. In such microscopic photographing device, such light shielding plate 23 having a pinhole 23a as is shown in FIG. 3 is arranged near any of the pupil position P.sub.0 of the objective lens 16 and its conjugate positions P.sub.1 and P.sub.2 so that its center may coincide with the optical axis and is rotated to focus the image.
In such conventional focusing device as is described in the above, there are disadvantages that, in case the magnification of the objective lens varies, with the variation of the size of the pupil, the pinhole 23a will be out of the pupil and the focusing will not be able to be made.